Softening of hard water by the removal of calcium values therefrom is required for both industrial and household use to decrease scale formation in equipment such as boilers, hot water pipes and appliances such as pots and kettles, and also for the decrease of detergent consumption in household machines such as dishwashers and laundry washing machines. Softening of water is, moreover, effected as a pre-treatment for water desalination.
The factors to be considered in the choice of a water softening process include the raw water quality, the end use and desired quality of the soft water for such use, the ways and costs of disposing the waste streams, ecological problems associated with the process in general and with the waste disposal in particular, and the versatility of the process and its adaptability to different processing scales.
In all known water softening processes in which calcium carbonate is precipitated lime or alkali such as, for example, sodium hydroxide has to be added to the feed hard water. To avoid the handling of excessively large volumes, concentrated aqueous alkali solutions, e.g. caustic soda solutions are used. However, the handling of concentrated caustic soda or other equivalent alkaline solutions is inconvenient and hazardous and may be unsuitable for various industrial and domestic applications. It is therefore an object of the present invention to provide an alternative process for softening hard water by the precipitation of calcium carbonate therefrom, which does not require the addition of an alkali.
Water desalination by electrodialysis has been practised for many years. Generally speaking, the process is carried out in an apparatus, often referred to as a stack, in which a plurality of cation-selective and anion-selective membranes are placed alternatingly to form diluate and concentrate compartments alternating with each other. In operation, salt anions and cations such as, for example Na.sup.+ and Cl.sup.-, are drawn from each diluate compartment to the flanking concentrate compartments, the cations into one and the anions into the other. The desired product desalted water is withdrawn from the diluate compartments while the brine withdrawn from the concentrate compartments is rejected.
It has already been suggested to use for electrodialysis so-called asymmetric bipolar membranes, i.e. membranes having each an anion-exchange layer and a cation-exchange layer of unequal permselectivity combined in a single unit. Membranes of this kind are described, for example, in U.S. Pat. No. 3,562,139 (Leitz). Leitz also describes a method and apparatus for desalination wherein alternatingly oriented bipolar ion exchange membranes define the individual chambers of an electrodialysis apparatus and wherein the anion exchange laminae of each pair of said membranes bound the diluate chambers and the cation-exchange laminae bound the concentrate chambers. Leitz describes an electrodialysis processes with periodic reversal of the current flow by which fouling the membranes might be suppressed.
U.S. Pat. No. 4,217,200 (Kedem et al.) and U.S. Pat. No. 4,569,747 (Kedem et al.) describe sealed electrodialysis cells comprising two different monopolar membrane units, one being a cation-exchange and the other anion-exchange membrane, attached with each other along the peripheral regions and provided with means for the ingress and egress of liquid. In accordance with the disclosure in these two patents such membrane bags are used as concentrate compartments in electrodialysis stacks.
It is the object of the present invention to provide a process and apparatus for the softening of hard water by the precipitation therefrom of calcium carbonate, using an electrodialytic water splitting process for the alkalinization of the hard feed water, thereby to bring about the precipitation of calcium carbonate therefrom.